This invention relates to the repair of a protective coating on an article, and, more particularly, to the treatment of a local, discrete portion of the coating.
Certain articles, such as components operating in the hotter gas path environment of gas turbine engines are subjected to significant temperature extremes in an oxidizing atmosphere and corrosive contaminants such as sulfur, sodium, calcium and chlorine which are present in the combustion gases. As a result of service operation in and exposure to such an environment, surfaces of components such as turbine blades and vanes are subject to various types of degradation, including oxidation degradation, which herein is intended to include within its meaning corrosion and sulfidation degradation. To protect the component substrate from excessive environmental attack, surfaces of such components normally are treated with a protective coating system including a metallic environmental resistant coating of a type widely reported in the gas turbine engine art. These environmental resistant coatings generally are classified as diffusion or overlay coatings, distinguished by the processing methods or degree of substrate consumption during deposition.
Reported metallic environmental resistant coatings sometimes are referred to as diffused aluminide coatings as well as overlay coatings. Such coatings rely on interdiffusion of an applied coating element such as Al with an element on which a substrate is based, for example Ni, to create an intermetallic surface layer. These coatings have been applied by a variety of methods including pack cementation, above the pack, vapor phase, chemical vapor deposition and slurry type coating. The thickness and aluminum content of the end product coating can be controlled by varying such coating parameters and materials as the coating source materials, coating time, coating temperature and aluminum activity. For example, such control is reported in a variety of U.S. Pat. No. including 3,544,348--Boone et al. (patented Dec. 1, 1970), and U.S. Pat. No. 5,658,614--Basta et al. (patented Aug. 19, 1997). The oxidation resistant performance of diffused aluminide coatings has been shown to be enhanced by incorporating at least one of Pt, Rh, Pd, Hf, Si, Y and Zr. To incorporate these elements, thin layers of such elements generally are deposited by electroplating or physical vapor deposition means, prior to the aluminide coating cycle.
It has been observed that, as a result of engine operation under such strenuous operating conditions, degradation has occurred at least in some local areas. Such degradation can include oxidation products substantially completely through the metallic environmental resistant coating into the metal substrate as well as production of defects in the metallic environmental coating. Forms of such defects sometimes are referred to as "rumpling" and as coating voiding, one example of which is bond coat voiding. Such degradation can extend not only into the protective coating, such as a bond coat or portions of a diffused aluminide, but also can extend completely through the protective coating into the substrate of the article.
Degradation of the original coating composition that occurs during engine service operation varies in intensity across surfaces of a turbine airfoil, with some surfaces experiencing very little attack or composition loss. However, a current repair practice includes the complete removal from all coated surfaces of the protective diffused coatings or overlay coatings prior to repair. The complete removal results in loss of wall thickness, from removal of the interdiffused region, which reduces component load bearing capability. Additionally, complete coating removal creates problems with maintenance of designed cooling air flow patterns and flow rates for air cooled components. Such problems can occur at points at which cooling holes, communicating with internal cooling passages, intersect the component outer surface from which the coating has been removed and must be replaced for component reuse.